let callbacks = []

function loadedTinymce() {
    // to fixed https://github.com/PanJiaChen/vue-element-admin/issues/2144
    // check is successfully downloaded script
    return window.tinymce
}

const dynamicLoadScript = (src, callback) => {
    const existingScript = document.getElementById(src)
    const cb = callback || function() {}

    if (!existingScript) {
        const script = document.createElement('script')
        script.src = src // src url for the third-party library being loaded.
        script.id = src
        document.body.appendChild(script)
        callbacks.push(cb)
        const onEnd = 'onload' in script ? stdOnEnd : ieOnEnd
        onEnd(script)
    }

    if (existingScript && cb) {
        if (loadedTinymce()) {
            cb(null, existingScript)
        } else {
            callbacks.push(cb)
        }
    }

    function stdOnEnd(script) {
        script.onload = function() {
            // this.onload = null here is necessary
            // because even IE9 works not like others
            this.onerror = this.onload = null
            for (const cb of callbacks) {
                cb(null, script)
            }
            callbacks = null
        }
        script.onerror = function() {
            this.onerror = this.onload = null
            cb(new Error('Failed to load ' + src), script)
        }
    }

    function ieOnEnd(script) {
        script.onreadystatechange = function() {
            if (this.readyState !== 'complete' && this.readyState !== 'loaded') return
            this.onreadystatechange = null
            for (const cb of callbacks) {
                cb(null, script) // there is no way to catch loading errors in IE8
            }
            callbacks = null
        }
    }
}

export default dynamicLoadScript
